The objective is to define the specific roles played by brain stem and motor cortical neurons in the initiation and control of head and limb movements. In order to realize this objective, single units will be recorded from the motor cortex (arm and leg areas) and brain stem (reticular formation and lateral vestibular nucleus) of monkeys while they are performing a learned repertoire of head, arm, and leg movements. Unit activity, movement parameters (position and pressure), and EMG activity will be recorded on FM tape and averaged on and off line (CAT400 computer) for each movement of the repertoire. Each unit's receptive field will be determined, if possible. For units well related to movement(s), the EMG activity will be compiled (2,000 spikes) to determine if a "direct" connection exists between the unit spikes and muscle activity. Microstimulation at recording sites will be done and evoked movements and EMG activity noted. When relatively large populations of units (c.150) from each area have been assembled from several monkeys, the data will be relevant to a number of different questions. Is the brain stem preferentially related to axialproximal musculature and the precentral motor cortex to distal musculature (Kuypers' hypothesis)? Do these two brain regions preferentially control fast and slow muscle fibers? How does each region participate in premovement "set"? What parameters of movement does each region control? How is each area related to locomotion? What are the afferent input-motor output relations for each area? Answers, even partial ones, to these questions are fundamental to understanding the motor system as a whole. In addition, they may have clinical relevance to stroke and a variety of movement disorders.